In gas turbine plants, it is customary to cool the air extracted from the compressor by means of water injection or external cooling, before it is supplied as cooling air for the cooling system of the turbine. In this case, this heat is largely lost from the system as a whole.
By contrast, as is known, in combined plants water cooling of the air is usually carried out in an air/water heat exchanger and the heat occurring as a result of the cooling of the cooling air is made re-utilizable. By means of feed pumps, the pressure on the water side is raised above the saturated steam pressure to avoid evaporation loss, and the water heated in the cooler is subsequently expanded in a low-pressure system in which it can evaporate out. In a modified solution, the heat exchanger is operated in parallel with an economizer of a heat recovery steam generator following the gas turbine group.
The air cooler is integrated as a forced-flow once-through heater into a combined power plant. Simpler regulation and higher efficiency, as compared with the abovementioned cooling of the gas turbine plants, are thereby achieved. FIG. 1, which corresponds to FIG. 1 of the publication initially mentioned, shows a combined power plant 40 with a gas and a steam turbine group. The gas turbine group consists of a compressor 1, of a following combustion chamber 2 and of a gas turbine 3 arranged downstream of the combustion chamber 2. A generator 4 ensuring current generation is coupled to the gas turbine 3. The intake air 5 sucked in by the compressor 1 is conducted, after compression, as compressed air 6 into the combustion chamber 2 and is mixed there with injected liquid and/or gaseous fuel 7. The fuel/air mixture which occurs is burnt. The hot gas 8 flowing out of the combustion chamber 2 is subsequently expanded in the gas turbine 3 so as to perform work. The exhaust gas 9 from the gas turbine 3 is thereafter utilized in a heat recovery steam generator 15 of the following steam circuit.
Since the thermal load on the combustion chamber 2 and on the gas turbine 3 is very high, a cooling of the thermally stressed assemblies, which is as effective as possible, must take place. This is carried out with the aid of an air cooler 10 which is a helical steam generator. The air cooler 10 has flowing through it a part quantity, extracted from the compressor 1, of compressed air 11 which is already to a great extent heated up. Heat exchange within the air cooler 10 takes place by means of the water part stream 12 flowing through the tubes of the helical steam generator. The compressed air 11 is therefore cooled on one side to an extent such that it is subsequently conducted as cooling air 13 to the assemblies to be cooled. The high-pressure cooler is illustrated as an example in FIG. 1. It extracts fully compressed air 11 at the outlet of the compressor 1, and its cooling air 13 is used for the cooling of assemblies in the combustion chamber 2 and in the highest pressure stage of the gas turbine 3. As an alternative to this, air of lower pressure may also be extracted from an intermediate stage of the compressor 1, said intermediate stage being used for cooling purposes in the corresponding pressure stage of the gas turbine 3.
On the other side, the water part stream 12 is heated in the cooling air cooler 10 to an extent such that the water evaporates. This steam 14 is then conducted, according to FIG. 1, into the superheater part of a heat recovery steam generator 15. It increases the fresh steam 16 by which the steam turbine 17 is acted upon and thus serves for improving the efficiency of the plant as a whole. During this normal operation of the power plant, the steam 14 generated in the cooling-air cooler 10 is thus utilized optimally in energy terms. It is likewise possible to admix the steam 14 directly with the fresh steam 16 or to conduct it to the combustion chamber 2 or to the gas turbine 3.
The exhaust gas 9 from the gas turbine 3, said exhaust gas still having a high calorific potential, flows through the heat recovery steam generator 15. By means of the heat exchange method, these convert the feed water 18 entering the heat recovery steam generator 15 to fresh steam 16 which then forms the working medium of the remaining steam circuit. The calorifically utilized exhaust gases thereafter flow as flue gas 19 to the open. The energy arising from the steam turbine 17 is converted into current via a further coupled generator 20. A multishaft arrangement is illustrated as an example in FIG. 1. Of course, single-shaft arrangements may also be selected, in which the gas turbine 3 and the steam turbine 17 run on one shaft and drive the same generator. The exhaust steam 21 from the steam turbine 17 is condensed in a water-cooled or air-cooled condenser 22. The condensate is then pumped, by means of a pump not illustrated here, into a feed water tank/deaerator, not shown in FIG. 1, which is arranged downstream of the condenser 22. The feed water 18 is subsequently pumped via a further pump into the heat recovery steam generator 15 to form a new throughflow or a part stream 12 of the water is supplied to the air cooler 10 via a regulating valve, not shown here.
Publication EP-A1-0 773 349 initially mentioned, then, proposes, in FIGS. 2 to 5 and the accompanying description parts, various types of air cooler which are particularly suitable for use in a combined power plant according to FIG. 1. In embodiments of FIGS. 2 to 4, the cooling air to be cooled is led in the vertically standing air cooler, on the inside, in a central tube from the bottom upward, past the helical tube bundle of the heat exchanger which is arranged in a pressure vessel, is deflected downward above the tube bundle and flows through the tube bundle from the top downward, at the same time discharging heat to the steam flowing in countercurrent (from the bottom upward) in the tube bundle. Cooled cooling air emerging from the tube bundle at the bottom is deflected once again and flows in the pressure vessel, on the outside, past the tube bundle upward, where it is extracted from the pressure vessel. Since, in these configurations of the air cooler, the inside of the outer wall of the pressure vessel is exposed solely to the already cooled cooling air, the outer wall can be designed at a comparatively low operating temperature, thus affording considerable advantages, for example, with regard to the wall thickness required. By contrast, there are the disadvantages that the overall air stream has to be deflected upward, that a large annular duct is required for the deflected overall air stream, and that the overhead outlet connection piece is not suitable for the turbine.
By contrast, in the embodiment of FIG. 5 of EP-A1-0 773 349, the second deflection of the cooling air to the outlet of the tube bundle is dispensed with, and the cooled air is extracted directly below the tube bundle from the pressure vessel which at the same time also forms the container for the tube bundle. This variant has various plant-related advantages, but has the disadvantage that the walls of the pressure vessel become too hot, because they are exposed, particularly in the upper region of the air cooler, directly to the uncooled air coming from the compressor.